1. Field of the Invention
The present invention relates to a nonaqueous electrolyte which improves overcharge safety and a lithium battery using the same.
2. Description of the Related Art
Due to the development of advanced electronic devices, there is an increasing demand for small, lightweight portable electronic devices and equipment. Thus, there is a need for batteries having high energy density characteristics so as to supply power to such devices and equipment. Research into such lithium batteries is being aggressively pursued.
A lithium battery generally comprises a cathode, an anode, an electrolyte, which provides a movement path for lithium ions, and a separator disposed between the cathode and the anode. Lithium batteries produce electrical energy by intercalation/deintercalation of the lithium ions during oxidation and reduction occurring at the cathode and the anode. However, where a battery is overcharged due to a malfunction of a charger, for example, which causes a sharp rise in voltage, excess lithium is precipitated at the cathode and excess lithium is intercalated into the anode. If both the cathode and the anode are thermally unstable, an organic solvent of the electrolyte is decomposed so as to cause rapid heat generation, such as thermal runaway. This malfunction adversely affects battery safety.
To overcome the foregoing problem, various attempts have been proposed to suppress battery overcharge by changing the composition of an electrolyte or by adding additives to the electrolyte. For example, U.S. Pat. No. 5,580,684 discloses a method of improving battery safety by increasing a self-extinguishing property of an electrolyte through the addition of phosphoric acid esters, such as trimethyl phosphate, tri(trifluoroethyl)phosphate or tri(2-chloroethyl)phosphate, to the electrolyte. U.S. Pat. No. 5,776,627 discloses a method of enhancing battery safety by preventing a migration of lithium by adding additives such as thiophene, biphenyl or furan to be polymerized in the event of a failure of a battery, and by allowing a safety vent of the battery to be easily opened by the gas produced by the additives. Similarly, methods for enhancing battery safety are disclosed in U.S. Pat. Nos. 5,763,119, 5,709,968 and 5,858,573 in which an electrolyte includes additives such as 1,2-dimethoxy-4-bromo-benzene, 2-chloro-p-xyline and 4-chloro-anisol, and 2,7-diacetyl thianthrene, respectively. Also, Japanese Patent Laid-Open Publication No. Hei 7-302614 discloses a battery protecting method in which overcharge current is consumed by forming a polymer using a ter-phenyl benzene compound.
However, such conventional additives may be polymerized even under normal operating conditions or produce a large amount of gas due to an oxidative decomposition, resulting in a swelling of a battery. Also, use of such conventional additives may deteriorate various battery performance characteristics, such as formation, standard capacity or cycle life characteristics, making it difficult to put the additives into practical application.